So far, dielectric ceramic compositions of a barium titanate system have widely been put into practical use as a high permittivity ceramic dielectrics. However, the dielectric ceramics of this system have large dielectric loss at high frequencies and poor linearity of the temperature change rate of dielectric constant. In addition, they have the disadvantage that the capacitance is considerably decreased by application of a direct current voltage.
On the other hand, extensive efforts have been made to solve these problems and have led to development of dielectric ceramics of a system SrTiO.sub.3 -Bi.sub.2 O.sub.3 -TiO.sub.2 -PbTiO.sub.3 -CaTiO.sub.3 -CuO (Japanese patent application laid-open No 53-29599) or a system SrO-CaO-Bi.sub.2 O.sub.3 -TiO.sub.2 -Pb.sub.3 O.sub.4 -MgO (Japanese patent application laid-open No. 57-92703).
The compositions of the former system SrTiO.sub.3 -Bi.sub.2 O.sub.3 -TiO.sub.2 -PbTiO.sub.3 -CaTiO.sub.3 -CuO possess high quality factor (Q=1/tan .delta.) at low frequencies of the order of 455 KHz, but they considerably decrease in the quality factor at high frequencies of not less than 1 MHz. It is therefore impossible with this system to produce ceramic dielectrics for new applications which are continually increasing the operating frequency range. In the latter system SrO-CaO-Bi.sub.2 O.sub.3 -TiO.sub.2 -Pb.sub.3 O.sub.4 -MgO, ceramic compositions may have a high dielectric constant of the order of 500. However, such compositions have a large temperature coefficient of dielectric constant of the order of -2000 ppm/.degree. C., and the change rate of capacitance with temperature is -13 % at +85.degree. C. It is also possible with the latter system to produce dielectric ceramic compositions having a small change rate of capacitance with temperature, but their dielectric constant is low and of the order of 300. In addition, since all the compositions of these systems contain PbO, there is an inevitable evaporation of lead during sintering, resulting in variations in dielectric characteristics of the products. Thus, the sintering of these compositions requires special precautions to minimize the evaporation of lead and that causes many problems in mass production.